1. Field of the Invention
The present invention relates to information coding, and more specifically to the baseband transcoding of digital information that is to be transmitted over a telephone line.
2. Description of the Related Art
Conventional baseband transcoding has two steps: a coding step that associates a predetermined symbol with a sequence of digital information, and a shaping step that associates an elementary pulse of predetermined duration with each coded symbol. Among the codes typically used is a first type known as 2B1Q (defined in the European standard ETSI ETR 080 and the American standard ANSI T1.601) and a second type known as 4B3T (defined in the European standard ETSI ETR 080 and the German standard FTZ 1 TR 220. The codes are used for the transmission of digital information within a digital network of universal character, such as an integrated services digital network ("ISDN") (defined in the European standard ETSI ETR 080 and the American standard ANSI T1.601.
The three main characteristics of ISDN are: (1) the capability of simultaneously employing telephone, telematic, and even video-communication services from one access; (2) subscriber-to-subscriber integral digital transmission with digital connections in the various switching centers; and (3) signaling in message mode only between the various functional entities (e.g., terminals, switches, and servers). The architecture of the local network attaching users to an ISDN network includes: on the subscriber side, a network digital terminal (TNR) to which the various user terminals are attached, and on the switch side, a line terminal (TL) that provides substantially the same functions as the TNR terminal. Between the TNR terminal and the TL terminal there is a subscriber line (U interface) that consists of a pair of copper cables whose characteristics (e.g., maximum length, useful bit rate, and error rate) are standardized.
The useful bit rate with ISDN is 144 Kbits/s, which is used by two "B" channels operating at 64 Kbits/s for transporting useful information and a "D" channel operating at 16 Kbits/s for transporting check bits. Thus, the subscriber line alone provides the so-called "narrow band" ISDN connection that allows access at the basic bit rate (2B+D) of 144 Kbits/s. By using codes such as the 4B3T code or the 2B1 Q code, it becomes possible to reduce the spectral band used for transmission.
Transmitters operating with either the 2B1 Q code or the 4B3T code are known. However, when using a particular baseband transcoding, it is necessary to use a network digital terminal containing a coder specific to the type of coding employed. Thus, changing the type of coding requires replacing the network digital terminal with another network digital terminal that is specifically adapted for use with the new code.